Originally Posted by Garryese 
Keith, your friends machine likely needs the idle mixture screw turned out 1/4 of a turn. Run the warmed up machine a few times. If the backfiring continues then repeat the 1/4 turn until it quits. If the total adjustment is more than 1 turn complete turn then the pilot jet needs cleaning or the float level needs adjustment.
On yours, I would pull the spark plug and check the color of the insulator. If it is white or light grey then you are running lean, which makes more power, but too much heat. Ideal color is light brown or tan. If the insulator is black then your rich, but I would leave it alone as you are not suffering from lack of power and the rich condition will help the engine run cooler, thus avoiding the Griz 660 studdering problems.
Keep in mind that some engines just make a bit more power than others. My stock non HO Sportsman 500 beats every stock 500HO sportsman I have raced; my engine is tight where it needs to be and loose where it needs to be.
The flashing light does not put the machine in a limp mode so don't sweat it too bad if the spark plug color is good. The light indicates you are hitting the red line, but these engines can handle more than the red line if they are loaded and it is not for long period of time.
Hey Garryese. You sound like the man i need to talk to. I have a 2004 Grizz with a bog I cannot get rid of. I had the 686 kit put in and the head ported and took it to have it Dyno jetted after to have it jetted right. After this it still has a bog. In high gear it bogs at 17kmph - 19 and if you go through this point slowly before you hit the throttle your fine but if you hit it it bogs. In mud it is almost immpossible to run in high because i need to keep it spinning above these speeds. In low the bog is at like 5 kmph but same if i am easy on the throttle and get past this there is lots of power. I talked to the dyno jet guy and we originally thought I could be alittle rich so I dropped the needle 1 and it got worse so I went 1 slot richer and it is better again but still there. I could go richer but when Look at my plug it is already black. I have done alot of the heat related fixs (wrapped my pipe, Engine ice, vented panels etc.) Just wondreing what you might think. Sorry this is so long but this is two years in the works. ?